Typically, a wireless communication system makes communications between a fixed base station (BS) and an MS via a single direct link, thus having low flexibility in configuring a wireless network. That is, it is difficult to configure a wireless network to provide services under a wireless environment experiencing fluctuating changes in traffic distribution or the number of required calls.
In this context, studies have been conducted on relaying data over multiple hops using relay stations (RSs) in the wireless communication system.
The relay service enables the wireless communication system to reconfigure a network fast according to a communication environment change and to efficiently operate the entire wireless network.
Also, the wireless communication system can provide a high-speed radio channel to an MS by establishing an RS between a BS and the MS. Therefore, the use of RSs provides a high-speed data channel to MSs in poor channel status at a cell boundary and expands cell coverage.
When communications are not in progress, an MS is placed in idle mode or sleep mode in order to reduce power consumption. As illustrated in FIG. 1, the idle mode or sleep mode of the MS is controlled by its serving BS. While the control is about the MS's operation in the sleep mode in the illustrated case of FIG. 1, it is a mere exemplary application. Hence, the same thing applies to the MS's operation in the idle mode.
FIG. 1 is a diagram illustrating a signal flow for a sleep-mode operation in a conventional wireless communication system.
Referring to FIG. 1, an MS 101 transmits a sleep request message requesting transition to the sleep mode to a BS 105 through an RS 103 in steps 111 and 113.
Upon receipt of the sleep request message, the BS 105 determines whether to approve the sleep mode transition. If it approves the sleep mode transition, the BS 105 determines management information required for the MS 101 to operate in the sleep mode. Then the BS 105 transmits a sleep response message including the management information to the MS 101 through the RS 103 in steps 115 and 117. The management information specifies a sleep mode transition time, an initial sleep duration, a maximum sleep duration, a listening duration, and a sleep duration extension rate.
The BS 105 checks when the MS 101 is supposed to enter the sleep mode based on the sleep mode management information. When the MS 101 transitions to the sleep mode, the BS 105 transmits a traffic indication message to the MS 101 through the RS 103, taking into account the listening duration information of the MS 101 in steps 123 and 125. The traffic indication message includes the presence or absence of data to be transmitted to the MS 101.
Upon receipt of the sleep response message, the MS 101 determines whether the BS 105 has approved the sleep mode transition. If the BS 105 has approved, the MS 101 sleeps during the initial sleep duration from the sleep mode transition time based on the sleep mode management information in step 119.
Then the MS 101 transitions to listening mode according to the listening duration information and monitors reception of the traffic indication message from the RS 103 in step 121. Upon receipt of the traffic indication message, the MS 101 determines whether data for the MS 101 exists from the traffic indication message.
As described above, when it does not communicate, the MS operates in the idle mode or sleep mode under the control of the BS in order to save power. During the idle-mode operation or the sleep-mode operation, the MS receives a downlink signal only during a listening period. Therefore, the BS transmits a paging signal or a traffic indication message to the MS based on the listening duration information of the MS, to thereby notify the presence or absence of data for the MS.
When a relay service is provided, the BS transmits a signal to the MS via the RS. The resulting processing delay of the RS may make the transmission timing of the BS asynchronous to the reception timing of the MS. That is, the MS may not receive the paging signal or the traffic indication message that the BS transmits according to the listening duration information of the MS, due to the processing delay of the RS.